In all kinds of communications systems, linearity is a basic and important specification for both transmitters and receivers of the communications systems. For the transmitters, amplifiers are important and necessary components. The communication distance, communication quality and standby time, etc. of the transmitters are heavily associated with the amplifiers.
Please refer to FIGS. 1 and 2. FIG. 1 is a schematic diagram of an amplifier 100. FIG. 2 is a diagram representing the amplitude distortion (i.e., AM-AM distortion) and phase distortion (i.e., AM-PM distortion) of the amplifier 100. The amplifier 100 is used to amplify the input signal Sin to generate an output signal Sout. The amplifier 100 comprises a resistor Ra and a bipolar junction transistor (BJT) T1. The emitter of the BJT T1 is coupled to a ground terminal GND. In FIG. 2, a curve 101 is used to represent the phase distortion of the amplifier 100, and another curve 102 is used to represent the amplitude distortion of the amplifier 100. The horizontal axis of the diagram shown in FIG. 2 represents the output power Pout of the amplifier 100, and the vertical axis represents the amplitude distortion and phase distortion of the amplifier 100. The unit of the amplitude distortion is “dB”, and the unit of the phase distortion is “degree”. As shown in FIG. 2, the curve 101 is a curve with an upward notch and the curve 102 is a curve with a downward notch so that the phase distortion of the amplifier 100 increases as the output power Pout increases and that the amplitude distortion of the amplifier 100 decreases as the output power Pout increases. However, since the notches of the curves 101 and 102 are curved toward different directions, it is difficult to compensate the linearity of the amplifier 100.